Previously, we found that Friend plan spleen focus forming virus (SFFV) encodes a membranes glycoprotein (gp55) which is structurally related to the larger env gene products of recombinant-type dualtropic MCF viruses, that gp55 causes mitosis of infected erythroblasts, and that the xeno- and eco- related portions of gp55 fold into separate globular domains connected by a proline-rich joint. Our recent results suggest that the xeno-related domain of plasma membranal gp55 contains an active site that binds to MCF receptors (and perhaps to other structurally related membrane components) to trigger erythroblast proliferation. Thus, a leukemogenic receptor may be related or identical to MCF receptors. To objectively study these issues, the following approaches are proposed: (i) Analyze additional site-directed SFFV env mutants that we have constructed. These cause different in-frame focal lesions. (ii) Previously, we found that SFFV env genes can change rapidly and that nonleukemogenic mutants often form revertants in vivo. Sets of revertants with second-site env mutations will be molecularly cloned and sequenced. In (i) and (ii), we will identify gp55 active sites and correlate pathogenesis with subcellular localization and with MCF interference. (iii) Soluble forms of gp55 will be made and their biological effects, including binding to cell surface receptors and MCF and growth factor interferences, will be studied. (iv) The xeno-related sequences of Friend SFFV will be substituted for the homologous sequences of a Friend MCF, and the recombinant will be analyzed for replication conpetency. (v) We constructed a pathogenic SFFV with an inserted "tag" of foreign nucleic acid. This will be used to study SFFV integration sites and leukemic progression in the absence of a helper virus. (vi) Using different strategies, identify cell surface receptor(s) for gp55 and molecularly clone receptor gene(s). Our finding that Friend erythroleukemia is a primary membrane abnormality deserves thorough investigation. Retroviral env gp's have been increasingly implicated in leukemogenesis and in immunosuppression. Our objective has broad relevance for cancer research: to understand how an abnormality of cell membranes can cause premalignant hyperplasia that progresses to overt cancer.